<?php

$xml = simplexml_load_file('users.xml');
$usr = $_POST['username'];
$pass = $_POST['pass'];
if(empty($usr)) {
    die('Please enter an username!');
} elseif(empty($pass)) {
    die('Please enter a password!');
}
$l_usr = null;
$pass_e = md5($pass);
foreach($xml->user->username as $username) {
    if($username == $usr) {
        $l_usr = $username;
    }
}
if(empty($l_usr)) {
    die('User Not Found!');
}
if(!$xml->user->$l_usr->password == $pass_e) {
    die('Wrong Password!!');
}

echo 'Hello, '.$l_usr;

<users>
<user>
    <username>doruk.ayar</username>
    <password>96b9c62c86f35c209c5b8c302ba34175</password>
</user>
<user>
    <username>mina.alpturer</username>
    <password>32250170a0dca92d53ec9624f336ca24</password>
</user>
<user>
    <username>efe.gormus</username>
    <password>32250170a0dca92d53ec9624f336ca24</password>
</user>